He was a law clerk to Judge Albert J. Engel Jr. of the United States Court of Appeals for the Sixth Circuit from 1982 to 1983.
From 1995 to 1996, Gardephe held the position of Senior litigation counsel, in the U.S. Attorney's Office, Southern District of New York.
He was also a consultant (special counsel) for the Office of the Inspector General of the United States Department of Justice from 1996 to 2000 and from 2001 to 2003.
He returned to private practice from 2003 to 2008 as partner at Patterson Belknap, eventually heading the litigation department.
Gardephe was confirmed by the United States Senate on July 17, 2008, and received his commission on August 8, 2008.